Electron amplifier



1940- K. SCHLESINGER 2,227,030

ELECTRON AMPLIFIER Filed Feb. 5, 1938 jnvemop I Patented Dec. 31, 1940 PATENT OFFICE ELECTRON ADIPLIFI'ER Kurt Schlesinger, Berlin, Germany, :assignor, by

mesneassignments, to Loewe Radio, Inc.,-a corporation of New York Application February 5,, 1938, Serial No.'1-88,951

In Germany February, 16, 1937 9 Claims.

Arrangements for the amplification of electronic current are Well known comprising a se-.

ries of nets of graduated potential, at which a current amplification takes place by secondary emission-so-caLlled net amplifiers. It has also already been proposed to include between these nets cylindrical rings of a more negative potential for the purpose of focusing the electrons leaving onenet on to the next net. Upon the assembly of current amplifying means of this kind having concentrating rings there occur, however, unforeseen difiiculties commencing at amplifications of approximately 10 which can readily be produced with, say, .9 nets and a total potential of approximately 2,000 volts.

Thesedifficulties are due to three effects, which are different in themselves:

1..Basic noise of the cathode,

2. Wall currents andleakage,

3. .Interferences by reason of defective interception of the end current at the collector.

These difficulties and the way how they are avoided may be best understood by reference to the following description taken .in connection with the accompanying drawing, in which- Fig. 1 shows a diagrammaticyiew of a net amplifier tube of the design hitherto usual and of its circuit connection whilst I Fig. 2 is a section through a tube according to the invention.

In. Figure 1, I is a photo-electrode which receives the light from the light source M, 2 are amplifying nets, and 3 are concentrating rings which concentrate the electronic current. Each of these rings is made to be more negative by one potential stage than theinitial net of the appertaining amplifying field. Within the last rings 3 the collecting electrode 5 is arranged which is connected to the output terminal Hi. If a net amplifier of this character is employed and the currents flowing into the primary cathode or the initial net are measured in the galvanometers g1 and g2, it will be ascertained that :in an arrangement of this nature as shown in Fig. 1 the current through g2 mostly will not only fail to exhibit any amplification as compared with the current g1, but frequently will even be very much weaker. On the other hand considerable currents are to be observed in the dark from the lighting up of the glass wall at 4, especially if it is coated with a luminous substance. The photo-electrons find an easy path beside the first electrodes to the electrodes nearer the most positive end of the series. These currents are lost so .far as the amplification is concerned. Finally von the collector electrode 5 brush discharges are to be observedcaused by secondary electrons which fail to be concentrated by the adjacent concentrating rings 53.

According to the invention Lfirst of all the 'following improvements are made:

.1. Between the cathode and the first amplifying .net the electrons are by electrical means so governed that only a very small percentage fails to :re'ach the ifirst net and the loss "of primary current is very low. a

2. ,By means of an electrostatic shield the penetration .of the high positive potentials into the first discharge space is considerably reduced.

"3. By means of special screening nets the collector electrode 5 is completely separated from the wall space, and'the emission of electrons from the collector is avoided by negatively biasing these screens.

fier designed on these lines. connections are drawn. The electron-optical concentration'o'f the primary currents takes place I by designing the photo-electrode I as a spheri cal cup, at the centre of which or in the vicinity thereof the first net 2 is situated. In this manner an effective bundling of the primary electrons on to the electrode 2 is obtained without the use of negatively biased annular electrodes,

be provided at a later point.

The same object of driving back stray electrons from the wall space 1 can also be accomplished .by a conductive layer coating 8b over the upper inner wall of the tube and being connected with the cathode .I. To avoid point disr .20 In Fig. .2 there is illustrated a current amp'l'i- Only the internal charges the lower end of the. wall coating is to I coated with copper or silver at I 00.

in, it will be observed immediately that the noise the electrons, in contradistinction to Fig. 1, are no longer focussed above and below by two separate concentrating cylinders 3, this electrode being situated within a cylinder II. In this way the emission of rapid secondary electrons into the wall space I is avoided.

Upon the operation of a net amplifier constructed on these lines two interferences still occur: (1) the high-tension interference, (2) the noise interference.

The high-tension interferences are caused by minute, parasitic discharges within the tube, the amplitudes of which in comparison of that of the signals are rather high, and must be carefully avoided. In manufacturing normal tubes barium carbonate is usually employed for coating glass surfaces. This material, which is nonconductive in itself, causes a roughening of the surface, thus increasing the leakage path. As the applicant, however, has found, for chemical reasons barium carbonate is unsuitable for discharge tubes activated with caesium. According to the,

invention the glass surface, for example that of the base and the rods supporting the system, 'is

gpreviously treated with an etching agent like --a ring l3.

hydrofluoric acid gas. Upon subsequent cleaning and drying the surface insulation of glass roughened in this manner is very much better than that of a smooth surface without disadvantages of any kind.

There remains still a noise caused by the fiuctuations of the thermionic current emitted by the cathode at room temperature and amplified by the tube system. According to the invention 7 this noise is suppressed by strongly cooling the cathode as well as the first two amplifying nets, e. g. by means of acid snow, liquid air, etc. For this purpose according to Fig. 2 the spherical glass body carrying the cathode layer I is constructed itself as Dewar vessel, H filled with liquid air. The vessel consists of a double-wall glass body having the. inner wall Ina-[0b and the outer wall Iiic. Whilst the cathode layer is made up on a silver coatingon the side Ilia of the inner wall, the side 10b directed away from the tube is coated with a photo-electricallyinactive metal, for example platinum engraving colour or copper. The entire glass member II! is sealed into the main vessel which may also be To all of the metallic coatings may be imparted cathode potential or a somewhat higher one. In the latter case their natural emission is harmless, as they have no anode. After the liquid air is filled is mostly frozen in. The rest comes apparently from the natural thermal emission of the first nets 2. According to the invention these nets can also be included in the cooling by employ- "ing good heat-conductive supports l2 made preferably of copper and having a sufficient diameter for supporting the first concentrating cylinder 3, and at the same time placing these supports in good thermal contact with the glass body [ID by 3 and l are in any case electrically connected. The cylinder 3 then exerts a cooling effect, particularly if the surface of the concentrating cylinder is designed as a black or approximately black body. There are then also ,of dealing with light currents down to a weakness which is such that the primary emission itself merely consists of very few and discontinuous electrons. The last rests of basic noise, of course, cannot be overcome by measures of any kind. It can merely be accomplished that in the case of black transmission, the noise disappears entirely which condition is sufficient for a good transmission.

I claim:

1. A photo-electric tube for multiplying photoelectric currents by means of secondary electron emission comprising a photo-cathode and a series of flat metallic nets adapted for secondary emission, said photo-cathode forming a part of a concave sphere and the first of said nets being mounted a distance therefrom approximately equal to the radius of said sphere, and a conductive coating on the inner wall of the tube connected with said cathode, and provided, at its end remote from the cathode, with a wire ring for protecting said wall coating against flashing.

2. A photo-electric tube for multiplying photoelectric currents by means of secondary electron emission comprising a photo-cathode and a series of fiat metallic nets adapted for secondary emission, said photo-cathode forming a part of a concave sphere, said sphere being the bottom of a vessel adapted to be filled with a fluid having a very low boiling point for cooling said cathode.

3. A photo-electric tube for multiplying photoelectric currents by means of. secondary electron emission comprising a photo-cathode, a series of flat metallic nets adapted for secondary emission, and a series of cylindrical electrodes for concentrating said current, said electrodes being mounted between said nets and the first of said elecode, said photo-cathode forming a part of a concave sphere, said sphere being the bottom of a vessel adapted to be filled with a fluid having a very low boiling point for cooling said cathode, and the first of said electrodes being also ther- .35 trodes being electrically connected with the cathmically connected with said cathode by means of rods of high thermal conductivity. 4. A photo-electric tube for photo-electric currents by means of secondary electron emission comprising a photo-cathode forming a part of a concave sphere, a multiplying system consisting of a series of flat metallic multiplying influences of the electrodes of the later stages of said multiplying system.

5. A photo-electric tube for multiplying photo-electric currents by means of secondary electron emission comprising a photo-cathode forming a part of a concave sphere, a multiplying system consisting of a series of fiat metallic nets adapted for secondary emission, a series of cylindrical concentrating electrodes and a terminal anode, said nets and said cylindrical electrodes being of substantially the same diameter, each of said cylindrical electrodes being mounted between two of said nets, and a shield electrode mounted between the first of said cylindrical electrodes and the tube wall for screening the discharge path between said photocathode and the first of said nets against undesired influences of the electrodes of the later stages of said multiplying system.

6. A photo-electric tube for multiplying photo-electric currents by means of secondary electron emission comprising a photo-cathode forming a part of a concave sphere, a multiplying system consisting of a series of flat metallic nets adapted for secondary emission, a series of cylindrical concentrating electrodes and a terminal anode, said nets and said cylindrical electrodes being of substantially the same diameter, each of said cylindrical electrodes being mounted between two of said nets, and a ring-shaped diaphragm surrounding said multiplying system, said diaphragm being co-axially mounted with said multiplying system and extending to the inner surface of the tube wall, for screening the discharge paths on its side facing said photocathode against undesired influences of the electrodes of the later stages of said multiplying system. i

7. A photo-electric tube for multiplying photo-electric currents by means of secondary electron emission comprising a photo-cathode forming a part of a concave sphere, a multiplying system consisting of a series of flat metallic nets adapted for secondary emission, a series of cylindrical concentrating electrodes and a terminal anode, said nets and said cylindrical electrodes being of substantially the same diameter,v each of said cylindrical electrodes being mounted between two of said nets, and a conductive wall coating electrically connected with said cathode.

8. A photo-electric tube for multiplying photo-electric currents by means of secondary electron emission comprising a primary photocathode, a series of secondary cathodes activated for secondary electron emission, and receptacle means supporting said photo-cathode for cooling said photo-cathode below room temperature.

9. A photo-electric tube for multiplying photo-electric currents by means of secondary electron emission comprising a primary photocathode and a series of secondary cathodes activated for secondary electron emission, the support of said photo-cathode forming a part of a vessel adapted to be filled with a substance having a very low boiling point for cooling said cathode.

' KURT SCI-ILElSINGER. 

